1. Field of the Invention
This invention relates generally to flyback transformer circuits used for a CRT (cathode ray tube), and particularly to such a circuit arranged to supply signal processing circuits with d.c. power obtained from additional winding or windings of a flyback transformer.
2. Prior Art
In a display apparatus or unit using a cathode ray tube, such as a television set or monitor display, which can be connected to various external apparatus, the power source which supplies the video and/or audio signal processing circuits or an interface to be connected to such an external apparatus should be isolated from a commercial a.c. power source.
Conventionally, a flyback transformer is used for effecting such isolation such that various pulse voltages obtained from one or more secondary windings of a flyback transformer are rectified and smoothed to be converted into a d.c. voltage which is then supplied to one or more signal processing circuits or interface. In such an arrangement, a high voltage secondary winding used for developing a high voltage to be fed to the anode of a CRT should also be isolated from the commercial a.c. power source unless the video signal processing circuit is isolated from CRT drive circuits connected to electron guns of the CRT. In such an arrangement of a flyback transformer whose high voltage secondary winding is isolated from the a.c. power source, flyback pulses applied to the primary winding of the flyback transformer are apt to enter the video and audio signal processing circuit as well as to external apparatus via the ground line because the ground line of a horizontal output circuit connected to the primary winding is coupled via a capacitor with ground lines of the video and audio signal processing circuits or interface and external circuits. As a result, pulse like noise caused from the flyback pulses enters into video and audio signal circuits located at the secondary winding side of the flyback transformer as well as one or more external apparatus connected to the flyback transformer circuit, to cause these circuits to malfunction or to suffer from noise in video and/or audio signals.
Especially, in the case of external apparatus, such as a video tape recorder, which is not synchronized with horizontal sync in the horizontal output circuit, pulse like noise caused from flyback pulses is apt to appear in a picture on the CRT screen.